1. Field of the Invention
The current disclosure relates to a constant data accessing system and, in particular, to a constant data accessing system managing a value of constant pool base address in a constant pool base register.
2. Description of the Related Art
MIPS32 uses a conventional constant accessing method that constructs a 32-bit constant by two 32-bit instructions, Load Upper Immediate (LUI) and Or Immediate (ORI). In FIG. 1, LUI carries the top-half of the 32-bit constant and ORI carries the bottom-half of the 32-bit constant. The code size for constructing the 32-bit constant is 8 bytes.
Another one related art is program-counter-relative data access. X86-64, ARM, and DEC VAX support this instruction type. When program-counter-relative data access is used, compiler places the constants in a constant pool before or after the subroutine that uses the constants. And then, use program-counter-relative instructions to access the constants.
FIG. 2 schematically illustrates an example that how to use program-counter-relative instructions. The upper part 11 shows a C computer program code that defines a simple subroutine operable to retrieve a global variable “global_var” to increment its value and to store it back to memory. The lower part 13 illustrates an ARM assembly code equivalent to the C computer program code.
In the assembly code, the data at address 0x100 is the initialized value zero of the global variable “global_var”. The address of the global variable “global_var” is a constant data put at address 0x00c. The address of the global variable “global_var” is loaded by the program-counter-relative load instruction at address 0x000. Therefore, the code size of accessing the constant data is 6-bytes.
However, when the code size of the subroutine is very large, the address distance from the program-counter-relative load instruction to the address of the constant also becomes longer. In this case, the instruction size of the program-counter-relative load instruction is 4 bytes. Therefore, in this case, the code size of accessing the constant data is 8-bytes.
In order to improve the conventional constant accessing, there is a need for a constant data accessing system and method thereof.